For quite some time, the benefits of utilizing lasers for medical surgery have been recognized. For example, lasers are used by physicians to perform delicate operations within the eye of a patient. A laser is desirable because it can be used to treat very localized regions. More particularly, since laser light is coherent, it can be focused quite accurately. In addition, the absorption characteristics of tissues within the eye vary as a function of laser wavelength. Thus, particular features in the eye can be treated by choosing the proper wavelength radiation.
In all laser surgical devices, a means must be provided to permit the operator to aim the laser beam prior to treatment. Once the laser beam has been properly aimed, the high-powered surgical beam is activated. The time during which the surgical beam is actuated will typically be much shorter than the time necessary for aiming the beam. As can be appreciated, for surgery in delicate areas such as the eye, the aiming beam must have much less power than the surgical beam.
In some laser surgical devices, the laser generates wavelengths which are not visible to the human eye and therefore cannot be seen by the operator. In these devices, a second, low power laser which emits a visible beam is provided to generate an aiming beam. The use of a second laser to provide an aiming beam creates a number of problems. More specifically, optical elements must be provided to ensure that the aiming beam exits the device along a path coaxial with the surgical beam. In this manner, the operator can insure that the surgical beam will be delivered to the same location at which the aiming beam had been focused.
In many laser surgical devices, the surgical laser itself generates wavelengths that are visible to the human eye. In this case, it is preferable to use an attenuated version of the surgical beam to aim the laser. As can be appreciated, if the same laser is used to generate both the aiming and surgical beam, there is no need to align two different lasers. Accordingly, in prior art devices where only one laser is used, a variety of different approaches have been used to attenuate the power of the surgical beam such that a weakened, low power version of the beam can be used for aiming.
The simplest form of a beam attenuator would be a neutral density filter that is selectively placed into the path of the high power surgical laser beam. A neutral filter would tend to absorb a portion of the surgical beam prior to its reaching the patient. The latter approach, however, is unsatisfactory for present surgical lasers. More specifically, since a neutral density filter performs the attenuation function by absorbing light energy, heating will occur in the filter. If the surgical beam is relatively powerful, this absorption will create a rapid temperature rise in the filter, causing it to crack.
Accordingly, where higher power surgical lasers are utilized, attenuating filters have been designed which include a thin, dielectric film layer. The dielectric film layer is specifically designed to reflect a large portion of the beam hitting its surface. The reflected portion of the beam is then directed to a beam dump which can safely absorb the laser power. The portion of the laser beam which passes through the dielectric film is highly attenuated. The plane of the filter is placed perpendicular to the path of the laser such that the attenuated beam passing through filter travels along the same path as the incoming beam. By this arrangement, the attenuated beam can be readily used as the aiming beam. When the filter is removed from the path of the beam, the full powered surgical beam will be delivered to the treatment area.
A beam initially attenuated by a dielectric film filter can also be passed through a neutral density filter located downstream from the dielectric film filter to further attenuate the beam. Since the beam has already been significantly reduced in power by the dielectric film filter, a downstream neutral density filter can be used since rapid overheating will not occur.
In practice, a device can be provided with a plurality of downstream neutral density filters of different strengths. By preselecting one filter from this group, the power of the aiming beam reaching the patient's eye can be varied over a wide range. The ability to select different powers of the attenuated beam is desirable since the amount of penetration of the aiming beam is related to its power. For example, if the surgeon is operating on a relatively clear eye, a very low power aiming beam may be sufficient. However, if the interior of the eye is clouded, a much higher power aiming beam will be necessary to penetrate the eye. Even in the latter case, the most powerful aiming beam will still be at least one thousand times weaker than the surgical beam.
In the photocoagulators marketed by Coherent, Inc., the assignee of the subject invention, a combination of dielectric film reflectors and neutral density filters have been utilized. This approach was quite satisfactory where the photocoagulator had a single type of laser emitting radiation over a narrow range. In this case, dielectric filter elements could be designed relatively easily to reflect the proper amount of radiation.
As pointed out above, it has been recognized that various types of eye surgery can be performed with different laser wavelengths. Accordingly, photocoagulators have been marketed which have included more than one type of laser to increase the range of wavelengths available to the surgeon. One type of prior art photocoagulator utilized both an argon ion laser and a krypton ion laser. An argon ion laser will generate significant output lines at 488 and 514 nanometers, while a krypton ion laser will generate significant power at wavelengths of 531, 568, 647, and 676 nanometers.
About one and one-half years ago, Coherent, Inc., introduced a new photocoagulator which used a combination argon ion laser and a dye laser. In this device, the output lines from the argon ion laser could be used to perform surgery. In addition, the argon laser beam could also be redirected by the internal optics of the device to pump the cavity in a dye laser. In this orientation, the beam from the dye laser could then be used for surgery. One of the key advantages of a dye laser is that its output is tuneable over a relatively wide range of wavelengths. For example, the dye laser used in Coherent's present photocoagulator generates radiation across spectrum from 577 to 630 nanometers. Additional background information relating to this laser system and in particular, the optical elements suitable for redirecting the beam of an argon laser to the cavity of the dye laser can be found in copending application, Ser. No. 881,135, filed July 1, 1986, incorporated herein by reference.
The above described argon/dye laser photocoagulator provides the surgeon with great flexibility in selecting a suitable wavelength radiation for treatment. However, the ability to generate such a wide spectrum of laser wavelengths increased the difficulty of designing suitable attenuation optics. More particularly, it was found to be very difficult to design and manufacture dielectric film filters which had constant reflectance characteristics over the entire range of laser wavelengths.
Initially, a number of different dielectric film filters were selectively used during operation of the prior art argon/dye laser photocoagulator. In the meantime, efforts were made to develop a beam attenuator which would be operative over a wide range of wavelengths. The result of such efforts produced the attenuator of the subject invention. It should be noted that while the subject beam attenuator was designed specifically for an argon/dye photocoagulator, it can also be utilized in place of other attenuation optical elements found in prior art surgical devices.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus for attenuating the power of a laser beam.
It is another object of the subject invention to provide a new and improved beam attenuator for use in a photocoagulator.
It is a further object of the subject invention to provide a new and improved apparatus which can be selectively placed in the path of a laser beam to produce an attenuated beam that travels along the same path as the incoming laser beam.
It is still another object of the subject invention to provide an apparatus which can be selectively moved into the path of a surgical laser beam in order to refract the main portion of the laser beam into a suitable beam dump.